1. Field of the Invention
The present invention relates to a multilayer directional coupler used in a mobile communication apparatus such as a cellular phone or the like.
2. Description of the Related Art
Multilayer directional couplers include, for example, the technologies disclosed in Japanese Unexamined Patent Application Publication No. 05-152814 and Japanese Unexamined Patent Application Publication No. 05-160614. These directional couplers have a structure in which strip-line transmission lines, each having a length of a quarter wavelength or less than a quarter wavelength, are laminated and formed in a dielectric substrate. On the basis of this structure, these directional couplers can be easily mass-produced and reduced in size. Accordingly, these multilayer directional couplers are widely used in mobile communication apparatuses, etc.
However, in these directional couplers, a bandwidth at which a stable degree of electromagnetic coupling can be obtained is represented by a bandwidth ratio of 50% or less. Therefore, these multilayer directional couplers cannot be used in apparatuses that need a signal having a bandwidth ratio of 90% or greater, such as a television signal.
Another known directional coupler, as shown in FIGS. 14 and 15, is capable of realizing a band with ratio of 99% or greater.
As shown in FIG. 14, this directional coupler 100 is constructed by winding an electrode 102 around a binocular-shaped magnetic core 101, and its circuit configuration is as shown in FIG. 15.
In this configuration, by appropriately setting a winding ratio (a ratio of the numbers of turns) N1 of a coil 112 to coil 111 of a transformer 110, and appropriately setting a winding ratio (a ratio of the numbers of turns) N2 of a coil 121 to coil 122 of a transformer 120, for example, a main signal input from a port 110a (port 110b) of the transformer 110 can be output to a port 110b (port 110a) and a port 120a (port 120b) of the transformer 120 at a distribution ratio corresponding to the winding ratios.
However, since, as shown in FIG. 14, for the directional coupler 100, an operation of manually winding the electrode 102 around the magnetic core 101 is needed, mass productivity is low compared with the above multilayer directional couplers. In addition, this type of directional coupler needs at least a volume of 80 mm3, and is accordingly not suitable in terms of size reduction. In addition, with the directional coupler 100, it is difficult to finely and accurately set the degree of electromagnetic coupling between the transformers because the degree of electromagnetic coupling between the transformers must be set on the basis of a winding ratio (a ratio of the numbers of turns) of the electrode 102. Accordingly, a main signal input to the port 110a (the port 110b) may not be output to the port 110b (the port 110a) and the port 120a (the port 120b) of the transformer 120 at a desired distribution ratio.